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A piece of melamine shelf stock (it's like $10 for a 2'x4' piece at the building supplies place) works fine as a base and seals well. You can't groove it, of course, but there are ways around that.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

So, I sealed everything real well with epoxy and now the press is pulling almost 20 inches. (about 17) I'll have to do a try run and see how much squeeze out I get out. I would like to get 22-25" but can't get it...

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

Is this correct or not? I once read that the most pressure you can get from a vacuum bag or a frame is really only 15 psi. because what is happening is all the air is being removed out of the bag or under the frame so you have the atmospherical pressure applied to it which is 15 psi. and to increase that the bag would have to be in a pressure chamber where the ambient pressure is higher then 15 psi. So then the inches would apply mostly to holding suction something like the guitars weight being held to one of the vacuum vises.

Chris, under optimal circumstances you want 25 inches or so for a gluing vacuum fixture. I'm sure yuo can get good results with less but I still prefer to be on the safe side...

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

Could you elaborate a bit more on why that is?

Chris, Both you and Peter are right as your using different units. Peters units are inches of mercury and you're talking pounds per square inch. 29.9ish inches of mercury is a full vacuum and is equal to 14.7 psi. So when peter says he got 17 inches that translates out to about 8.5 psi. Inches of Hg are a pretty standard way of measuring coarse vacuums.

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Jim Watts
http://jameswattsguitars.com

Vacuum can also be affected by the altitude at which you're running the pump. I'm no expert at all the engineering, and I've always lived within 150' or so of sea level so it hasn't affected my operations/equipment specs.
There is a tremendous amount of tech info an the Gast website. As said in a previous vacuum thread Gast is the gold standard of vac pumps.
http://www.gastmfg.com/technical.html
You can certainly pull 25 inches hg with a clamping rig. I do it routinely with a bag and one of the venturi type vac units.
-C

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Freeborn Guitars
and home of BeauGuard©

I can pull 25+ inches on all my vacuum fixtures but I guess the vacuum press still leaks....

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

If your subplate is truly airtight (plastic, etc...MDF or wood can and will will leak vacuum right through) then you can look for holes in your frame sealing or problems with gasket. Often there can be issues where your gasket pieces butt up against each other leaving a small crack for air to leak through. You can find leaks with your ears.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

My base is MDF sealed with epoxy...

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

If you're sure the base is well sealed, then I'd bet on a gasket problem. You can hear gasket problems if you've got sensitive ears. If you're not sure if it's the base or not then stick it to a known nonporous surface like the front of your fridge or the hood of your car or something and see if you can pull 25. If you can't it's a gasket issue or a sealing issue with your frame.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

How much holding power does the vacuum have? I'm interested in making some jigs say for shaping bridges and I was wondering if the vacuum would be strong enough to hold a bridge blank down in place while doing sanding or routing opperations on it. I would have pieces to keep it from moving side ways. The vacuum base would be corian as I have lots of it to make stuff from and I'm trying to think of uses for it.

Chris,
if you use some pins to keep the bridge from moving the vacuum with have more than enough holding power. In fact, you may even be able to do it without any pieces to keep it from moving side ways.

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

I cut my bridges on a little cnc router and use vacuum to hold the blanks. But that's not a big ole shaper cutter hogging off lots of material at once either. For milling operations vacuum works well on bridge sized pieces.
I use o-ring cord stock in a groove for my seals. One of the key things in making sure parts don't move on the fixture is getting the depth of groove just right. You want the part to come into contact with the fixture, not setting up on the seal where it can wobble around. If the grooves too deep you won't get a good seal.
The right depth of your groove depends on the material and size of your cord stock, also how much vacuum your pulling. It takes some trial.

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Jim Watts
http://jameswattsguitars.com

Thanks for the tips guys.

I built a venturi type vacuum press prototype based on info I found at this site :
http://www.kennethmichaelguitars.com/vacuumclamp.html
THe site has pictures and desription of the process. My (venturi type) prototype was sucessfull but I wanted to build something that I could just drop my radius dishes in for the radius I was using for back or top. When I built the bigger model (venturi type) I experienced much frustration becuase it would not seal. I used melamine at first. It didn't work well. I had all kinds of issues trying to get the membrane to adhere to the melamine. Then I built another using mdf sealed with polyurathane (sp?). It still would not seal. I asked around the OLF forum and the veneer forums and got several suggestions that seemed to point to the fact that I was using venturi vacuum method and it might not be up to the task of pulling vacuum over 26" that I was using.

I finally ended up with polyurathane sealed mdf with the sealing tape material from joewoodworkers site http://www.veneersupplies.com/product_i ... ts_id=1147 and a gast vacuum pump from the joewoodworkers site. I used the rubbberized material from LMI for the membrane and it works like a charm now. I built it based on pictures I had seen on Sylvan Wells site. He was also helpful in answering a few questions I had about how he'd built his. My top is unattached (no hinges) and I simply press down for a few seconds till it starts to seal.

So what was the problem? The gasket, pump or design?

I think the problem is a not properly sealed frame...

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com